Network Working Group I. Johansson
Internet-Draft Ericsson AB
Intended status: Standards Track K. Jung
Expires: June 18, 2011 Samsung Electronics Co., Ltd.
Dec 15, 2010
Negotiation of Generic Image Attributes in SDPdraft-ietf-mmusic-image-attributes-10
Abstract
This document proposes a new generic session set up attribute to make
it possible to negotiate different image attributes such as image
size. A possible use case is to make it possible for a low-end hand-
held terminal to display video without the need to rescale the image,
something that may consume large amounts of memory and processing
power. The draft also helps to maintain an optimal bitrate for video
as only the image size that is desired by the receiver is
transmitted.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 18, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Internet-Draft Image Attributes in SDP Dec 20101. Introduction
This document proposes a new attribute to make it possible to
negotiate different image attributes such as image size. The term
image size is defined here as it may differ from the physical screen
size of for instance a hand-held terminal. As an example it may be
beneficial to display a video image on a part of the physical screen
and leave space on the screen for other features such as menus and
other info.
There are a number of benefits with a possibility to negotiate the
image size:
o Less image distortion: Rescaling of images introduces additional
distortion, something that can be avoided (at least on the
receiver side) if the image size can be negotiated.
o Reduced receiver complexity: Image rescaling can be quite
computation intensive. For low end devices this can be a problem.
o Optimal quality for the given bitrate: The sender does not need to
encode an entire CIF (352x288) image if only an image size of
288x256 is displayed on the receiver screen. This alternatively
gives a saving in bitrate.
o Memory requirement: The receiver device will know the size of the
image and can then allocate memory accordingly.
o Optimal aspect ratio: The indication of the supported image sizes
and aspect ratio allows the receiver to select the most
appropriate combination based on its rescaling capabilities and
the desired rendering. For example, if a sender proposes three
resolutions in its SDP offer, 100x200, 200x100 and 100x100 with
sar=1.0 (1:1) etc. then the receiver can select the option that
fits the receiver screen best.
In cases where rescaling is not implemented (for example, rescaling
is not mandatory to implement in H.264 [H.264]), the indication of
the image attributes may still provide an optimal use of bandwidth
because the attribute will anyway give the encoder a better
indication about what image size is preferred and will thus help to
avoid wasting bandwidth by encoding with an unnecessarily large
resolution.
For implementers that are considering rescaling issues, it is worth
notice that there are several benefits to do it on the sender side:
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o Rescaling on the sender/encoder side is likely to be easier to do
as the camera related software/hardware already contains the
necessary functionality for zooming/cropping/trimming/sharpening
the video signal. Moreover, rescaling is generally done in RGB or
YUV domain and should not depend on the codecs used.
o The encoder may be able to encode in a number of formats but may
not know which format to choose as, without the image attribute,
it does not know the receiver's performance or preference.
o The quality drop due to digital domain rescaling using
interpolation is likely to be lower if it is done before the video
encoding rather than after the decoding especially when low
bitrate video coding is used.
o If low-complexity rescaling operations such as simple cropping
must be performed, the benefit with having this functionality on
the sender side is that it is then possible to present a miniature
"what you send" image on the display to help the user to target
the camera correctly.
Several of the existing standards ([H.263], [H.264] and [MPEG-4])
have support for different resolutions at different framerates. The
purpose of this document is to provide for a generic mechanism and is
targeted mainly at the negotiation of the image size but to make it
more general the attribute is named 'imageattr'.
This document is limited to point-to-point unicast communication
scenarios. The attribute may be used in centralized conferencing
scenarios as well but due to the abundance of configuration options
it may then be difficult to come up with a configuration that fits
all parties.
1.1. Requirements
The design of the image attribute is based on the following
requirements which are listed only for informational purposes:
REQ-1: Support the indication of one or more set(s) of image
attributes that the SDP endpoint wish to receive or send. Each
image attribute set must include a specific image size.
REQ-2: Support set up/negotiation of image attributes, meaning that
each side in the Offer/Answer should be able to negotiate the
image attributes it prefers to send and receive.
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REQ-3: Interoperate with codec specific parameters such as sprop-
parameter-sets in [H.264] or config in [MPEG-4].
REQ-4: Make the attribute generic with as few codec specific
details/tricks as possible in order to be codec agnostic.
Besides the above mentioned requirements, the requirement below may
be applicable.
OPT-1: The image attribute should support the description of image-
related attributes for various types of media, including video,
pictures, images, etc.
2. Conventions, Definitions and Acronyms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Specification of the 'imageattr' SDP Attribute
This section defines the SDP image attribute 'imageattr', which can
be used in an SDP Offer/Answer exchange to indicate various image
attribute parameters. In this document, we define the following
image attribute parameters: image resolution, sample aspect ratio
(sar), allowed range in picture aspect ratio (par) and the preference
of a given parameter set over another (q). The attribute is
extensible and guidelines for defining additional parameters are
provided in Section 3.2.10.
3.1. Attribute syntax
In this section the syntax of the 'imageattr' attribute is described.
The 'imageattr' attribute is a media-level attribute. The section is
split up in two parts, the first gives an overall view of the syntax
while the second describes how the syntax is used.
3.1.1. Overall view of syntax
The syntax for the image attribute is in ABNF [RFC5234]:
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----
onetonine = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"
; Digit between 1 and 9
xyvalue = onetonine *5DIGIT
; Digit between 1 and 9 which is
; followed by 0 to 5 other digits
step = xyvalue
xyrange = ( "[" xyvalue ":" [ step ":" ] xyvalue "]" )
; Range between a lower and an upper value
; with an optional step, default step = 1
; The rightmost occurrence of xyvalue MUST have a
; higher value than the leftmost occurrence.
/ ( "[" xyvalue 1*( "," xyvalue ) "]" )
; Discrete values separated by ','
/ ( xyvalue )
; A single value
spvalue = ( "0" "." onetonine *3DIGIT )
; Values between 0.1000 and 0.9999
/ ( onetonine "." 1*4DIGIT )
; Values between 1.0000 and 9.9999
srange = ( "[" spvalue 1*( "," spvalue ) "]" )
; Discrete values separated by ','.
; Each occurrence of spvalue MUST be
; greater than the previous occurrence.
/ ( "[" spvalue "-" spvalue "]" )
; Range between a lower and an upper level (inclusive)
; The second occurrence of spvalue MUST have a higher
; value than the first
/ ( spvalue )
; A single value
prange = ( "[" spvalue "-" spvalue "]" )
; Range between a lower and an upper level (inclusive)
; The second occurrence of spvalue MUST have a higher
; value than the first
qvalue = ( "0" "." 1*2DIGIT )
/ ( "1" "." 1*2("0") )
; Values between 0.00 and 1.00
----
o The attribute typically contains a "send" and a "recv" keyword.
These specify the preferences for the media once the session is
set up, in the send and receive direction respectively from the
point of view of the sender of the session description. One of
the keywords ("send" or "recv") MAY be omitted, see Section 3.2.4
and Section 3.2.2 for a description of cases when this may be
appropriate.
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o The "send" keyword and corresponding attribute list (attr-list)
MUST NOT occur more than once per image attribute.
o The "recv" keyword and corresponding attribute list (attr-list)
MUST NOT occur more than once per image attribute.
o PT is the payload type number, it MAY be set to "*" (wild card) to
indicate that the attribute applies to all payload types in the
media description.
o For sendrecv streams both of the send and recv directions SHOULD
BE present in the SDP.
o For inactive streams it is RECOMMENDED that both of the send and
recv directions are present in the SDP.
3.1.1.1. Parameter rules
For the parameters the following rules apply.
Payload type number (PT): The image attribute is bound to a specific
codec by means of the payload type number. A wild card (*) can be
specified for the payload type number to indicate that it applies
to all payload types in the media description. Several image
attributes can be defined for instance for different video codec
alternatives conditioned that the payload type number differs.
Note that the attribute is associated to the codec(s), for
instance an SDP offer may specify payload type number 101 while
the answer may specify 102, this may make it troublesome to
specify a payload type number with the 'imageattr' attribute. See
Section 3.2.2 for a discussion and recommendation how this is
solved.
Preference (q): The preference for each set is 0.5 by default,
setting the optional q parameter to another value makes it
possible to set different preferences for the sets. A higher
value gives a higher preference for the given set.
sar: The sar parameter specifies the sample aspect ratio associated
to the given range of x and y values. The sar parameter is
defined as dx/dy where dx and dy is the physical size of the
pixels. Square pixels gives a sar=1.0. The parameter sar MAY be
expressed as a range or as a single value.
If this parameter is not present a default sar value of 1.0 is
assumed.
The interpretation of sar differs between the send and the receive
directions.
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* In the send direction it defines a specific sample aspect ratio
associated to a given x and y image size (range).
* In the recv direction sar expresses that the receiver of the
given medium prefers to receive a given x and y resolution with
a given sample aspect ratio.
See Section 3.2.5 for a more detailed discussion.
The sar parameter will likely not solve all the issues that are
related to different sample aspect ratios but it can help to solve
them and reduce aspect ratio distortion.
The response MUST NOT include a sar parameter if there is no
acceptable value given. The reason to this is that if the
response includes a sar parameter it is interpreted as "sar
parameter accepted" while removal of the sar parameter is treated
as "sar parameter not accepted", for this reason it is safer to
remove an unacceptable sar parameter altogether.
par: The par (width/height = x/y ratio) parameter indicates a range
of allowed ratios between x and y physical size (picture aspect
ratio). This is used to limit the number of x and y image size
combinations, par is given as
----
par=[ratio_min-ratio_max]
----
Where ratio_min and ratio_max are the min and max allowed picture
aspect ratios.
If sar and the sample aspect ratio that the receiver actually uses
in the display are the same (or close), the relation between the x
and y pixel resolution and the physical size of the image is
straightforward. If however sar differs from the sample aspect
ratio of the receiver display this must be taken into
consideration when the x and y pixel resolution alternatives are
sorted out.
3.1.1.2. Offer/answer rules
In accordance with [RFC3264], offer answer exchange of the image
attribute is as follows.
o Offerer sending the offer:
* The offerer must be able to support the image attributes that
it offers. The exception is if the offerer has expressed a
wild card (*) in the attribute list.
* It is recommended that a device which sees no reason to use the
image attribute, anyway includes the attribute with wild cards
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(*) in the attribute lists for the send and recv directions.
An example of this looks like:
----
a=imageattr:97 send * recv *
----
This gives the answerer the possibility to express its
preferences. The use of wild cards introduces a risk that the
message size can increase in an uncontrolled way. To reduce
this risk these wild cards SHOULD only be replaced by an as
small set as possible.
o Answerer receiving the offer and sending the answer:
* The answerer may choose to keep the image attribute but is not
required to do so.
* The answerer may, for its receive and send direction, include
one or more entries that it can support from the set of entries
proposed in the offer.
* The answerer may also, for its receive and send direction,
replace the entries with a complete new set of entries
different from the original proposed by the offerer. The
implementor of this feature should however be aware that this
may cause extra offer/answer exchanges.
* The answerer may also remove its send direction completely if
it is deemed that it cannot support any of the proposed
entries.
* The answerer should not include an image attribute in the
answer if it was not present in the offer.
o Offerer receiving the answer:
* If the image attribute is not included in the SDP answer the
offerer SHOULD continue to process the answer as if this
mechanism had not been offered.
* If the image attribute is included in the SDP answer but none
of the entries are usable or acceptable, the offerer MUST
resort to other methods to determine the appropriate image
size. In this case the offerer must also issue a new offer/
answer without the image attribute to avoid misunderstandings
between offerer and answerer. This will avoid the risk on
infinite negotiations.
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Internet-Draft Image Attributes in SDP Dec 20103.2. Considerations3.2.1. No imageattr in 1st offer
When the initial offer does not contain the 'imageattr' attribute,
the rules in Section 3.1.1.2 require the attribute to be absent in
the answer. The reasons for this are:
o The offerer of the initial SDP is not likely to understand the
image attribute if it did not include it in the offer, bearing in
mind that Section 3.1.1 recommends that the offerer provide the
attribute with wild carded parameters if it has no preference.
o Inclusion of the image attribute in the answer may come in
conflict with the rules in Section 3.1.1.2 esp. the rules that
apply to "offerer receiving the answer".
For the above reasons it is RECOMMENDED that a device which sees no
reason to use the image attribute, anyway includes the attribute with
wild cards (*) in the attribute lists for the send and recv
directions.
3.2.2. Different payload type numbers in offer and answer
In some cases the answer may pick a different media payload type
number than the offer. As an example the offer SDP may have the
m-line
----
m=video 49154 RTP/AVP 99
----
While the answer SDP may have the m-line
----
m=video 49154 RTP/AVP 100
----
If the image attribute in the offer specifies payload type number 99,
this attribute will then have no meaning in the answerers receive
direction as the m-line specifies media payload type number 100.
There are a few ways to solve this
1. Use a wild card "*" as payload type number in the image attribute
in the offer SDP. The answer SDP also use the wild card. The
drawback with this approach is that this attribute then applies
to all payload type numbers in the media description.
2. Specify a wild card "*" as payload type number in the image
attribute in the answer SDP. The offer SDP may contain a defined
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payload type number in the image attribute but the answer SDP
replaces this with a wild card. The drawback here is similar to
what is listed above.
3. The image attribute is split in two parts in the SDP answer. For
example the offer SDP (only the parts of interest in this
discussion) looks like:
----
m=video 49154 RTP/AVP 99
a=imageattr:99 send ... recv ...
----
The answer SDP looks like:
----
m=video 49154 RTP/AVP 100
a=imageattr:99 send ...
a=imageattr:100 recv ...
----
This alternative does not pose any drawbacks. Moreover it allows
to specify different image attributes if more than one payload
type is specified in the offer SDP.
Of the alternatives listed above, the last one is RECOMMENDED to be
used if the media payload type number changes in the SDP answer.
3.2.3. Asymmetry
While the image attribute supports asymmetry there are some
limitations to this. One important limitation is that the codec
being used can only support up to a given maximum resolution for a
given profile level.
As an example H.264 [H.264] with profile level 1.2 does not support
higher resolution than 352x288 (CIF). The offer/answer rules imply
that the same profile level must be used in both directions. This
means that in an asymmetric scenario where Alice wants an image size
of 580x360 and Bob wants 150x120, profile level 2.2 is needed in both
directions even though profile level 1 would have been sufficient in
one direction.
Currently, the only solution to this problem is to specify two
unidirectional media descriptions. Note however that the asymmetry
issue for the H.264 codec is solved by means of the level-asymmetry-
allowed parameter in [RFC3984bis].
3.2.4. sendonly and recvonly
If the directional attributes a=sendonly or a=recvonly are given for
a medium, there is of course no need to specify the image attribute
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for both directions. Therefore one of the directions in the
attribute may be omitted. However it may be good to do the image
attribute negotiation in both directions in case the session is
updated for media in both directions at a later stage.
3.2.5. Sample aspect ratio
The relationship between the sar parameter and the x and y pixel
resolution deserves some extra discussion. Consider the offer from
Alice to Bob (we set the recv direction aside for the moment):
----
a=imageattr:97 send [x=720,y=576,sar=1.1]
----
If the receiver display has square pixels the 720x576 image would
need to be rescaled to for example 792x576 or 720x524 to ensure a
correct image aspect ratio. This in practice means that rescaling
would need to be performed on the receiver side, something that is
contrary to the spirit of this draft.
To avoid this problem Alice may specify a range of values for the sar
parameter like:
----
a=imageattr:97 send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
----
Meaning that Alice can encode with any of the mentioned sample aspect
ratios, leaving to Bob to decide which one he prefers.
3.2.6. SDPCapNeg support
The image attribute can be used within the SDP Capability Negotiation
[RFC5939] framework and its use is then specified using the "a=acap"
parameter. An example is
----
a=acap:1 imageattr:97 send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
----
For use with SDP Media Capability Negotiation extension
[SDPMedCapNeg], where it is no longer possible to specify payload
type numbers, it is possible to use the parameter substitution rule,
an example of this is.
----
...
a=mcap:1 video H264/90000
a=acap:1 imageattr:%1% send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
...
----
Where %1% maps to media capability number 1.
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It is also possible to use the a=mscap attribute like in the example
below.
----
...
a=mcap:1 video H264/90000
a=mscap:1 imageattr send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
...
----
3.2.7. Interaction with codec parameters
As the SDP for most codecs already specifies some kind of indication
of, for example, the image size, at session set up, measures must be
taken to avoid conflicts between the image attribute and this already
existing information.
The following subsections describe the most well known codecs and how
they define image-size related information. Section 3.2.7.4 outlines
a few possible solutions, but this document does not make any
recommendation for any of them.
3.2.7.1. H.263
The payload format for H.263 [H.263] is described in [RFC4629].
H.263 defines (on the fmtp line) a list of image sizes and their
maximum frame rates (profiles) that the offerer can receive. The
answerer is not allowed to modify this list and must reject a payload
type that contains an unsupported profile. The CUSTOM profile may be
used for image size negotiation but support for asymmetry requires
the specification of two unidirectional media descriptions using the
sendonly/recvonly attributes.
3.2.7.2. H.264
The payload format for H.264 [H.264] is described in [RFC3984] and
updated in [RFC3984bis].
H.264 defines image size related information in the fmtp line by
means of sprop-parameter-sets. According to the specification
several sprop-parameter-sets may be defined for one payload type.
The sprop-parameter-sets describe the image size (+ more) that the
offerer sends in the stream and need not be complete. This means
that this does not represent any negotiation. Moreover an answer is
not allowed to change the sprop-parameter-sets.
This configuration may be changed later inband if for instance image
sizes need to be changed or added.
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Internet-Draft Image Attributes in SDP Dec 20103.2.7.3. MPEG-4
The payload format for MPEG-4 [MPEG-4] is described in [RFC3016].
MPEG-4 defines a config parameter on the fmtp line which is a
hexadecimal representation of the MPEG-4 visual configuration
information. This configuration does not represent any negotiation
and the answer is not allowed to change the parameter.
It is not possible to change the configuration using inband
signaling.
3.2.7.4. Possible solutions
The subsections above clearly indicate that this kind of information
must be aligned well with the image attribute to avoid conflicts.
There are a number of possible solutions, listed below without any
preference:
o Ignore payload format parameters: This may not work well in the
presence of bad channel conditions especially in the beginning of
a session. Moreover this is not a good option for MPEG-4.
o 2nd session-wide offer/answer round: In the 2nd offer/answer the
codec payload format specific parameters are defined based on the
outcome of the 'imageattr' negotiation. The drawback with this is
that set up of the entire session (including audio) may be delayed
considerably, especially as the 'imageattr' negotiation can
already itself cost up to two offer/answer rounds. Also the
conflict between the 'imageattr' negotiation and the payload
format specific parameters is still present after the first offer/
anser round and a fuzzy/buggy implementation may start media
before the second offer/answer is completed with unwanted results.
o 2nd session-wide offer/answer round only for video: This is
similar to the alternative above with the exception that set up
time for audio is not increased, moreover the port number for
video is set to 0 during the 1st offer answer round to avoid that
media flows.
This has the effect that video will blend in some time after the
audio is started (up to 2 seconds delay). This alternative is
likely the most clean-cut and failsafe alternative. The drawback
is, as the port number in the first offer is always zero, the
media startup will always be delayed even though it would in fact
have been possible to start media already after the first offer/
answer round.
Note that according to [RFC3264], a port number of zero means that
the whole media line is rejected meaning that a new offer for the
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same port number should be treated as a completely new stream and
not as an update. The most safe way to solve this problem is to
use preconditions, this is however outside the scope of this
document.
3.2.8. Change of display in middle of session
A very likely scenario is that a user switches to another phone
during a video telephony call or plugs a cellphone into an external
monitor. In both cases it is very likely that a renegotiation is
initiated using the SIP-REFER or SIP-UPDATE methods. It is
RECOMMENDED to negotiate the image size during this renegotiation.
3.2.9. Use with layered codecs
As the image attribute is a media level attribute, its use with
layered codecs cause some concern. If the layers are transported in
different RTP streams the layers are specified on different media
descriptions and the relation is specified using the grouping
framework [RFC5888] and the depend attribute [RFC5583]. As it is not
possible to specify only one image attribute for several media
descriptions the solution is either to specify the same image
attribute for each media description, or to only specify the image
attribute for the base layer.
3.2.10. Addition of parameters
The image attribute allows for the addition of parameters in the
future. To make backwards adaptation possible; an entity that
processes the attribute MUST ignore any unknown parameters in the
offer and MUST NOT include them in the answer it generates. Addition
of future parameters that are not understood by the receiving
endpoint may lead to ambiguities if mutual dependencies between
parameters exist, therefore addition of parameters must be done with
great care.
4. Examples
This section gives some more information on how to use the attribute
by means of a high-level example and a few detailed examples.
4.1. A High-Level Example
Assume that Alice wishes to set up a session with Bob and that Alice
takes the first initiative. The syntactical white-space delimiters
(1*WSP) and double-quotes are removed to make reading easier.
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In the offer Alice provides information for both the send and receive
(recv) directions. For the send direction Alice provides a list that
the answerer can select from. For the receive direction Alice may
either specify a desired image size range right away or a * to
instruct Bob to reply with a list of image sizes that Bob can support
for sending. Using the overall high level syntax the image attribute
may then look like
----
a=imageattr:PT send attr-list recv attr-list
----
or
----
a=imageattr:PT send attr-list recv *
----
In the first alternative the recv direction may be a full list of
desired image size formats. It may however (and most likely) just be
a list with one alternative for the preferred x and y resolution.
If Bob supports an x and y resolution in at least one of the X and Y
ranges given in the send attr-list and in the recv attr-list of the
offer, the answer from Bob will look like:
----
a=imageattr:PT send attr-list recv attr-list
----
And the offer answer negotiation is done. Worth notice here is that
the attr-list will likely be pruned in the answer. While it may
contain many different alternatives in the offer it may in the end
contain just one or two alternatives.
If Bob does not support any x and y resolution in one of the provided
send or recv ranges given in the send attr-list or in the recv attr-
list, the corresponding part is removed completely. For instance, if
Bob doesn't support any of the offered alternatives in the recv attr-
list in the offer, the answer from Bob would look like:
----
a=imageattr:PT recv attr-list
----
4.2. Detailed Examples
This section gives a few detailed examples, it is assumed where
needed that Alice initiates a session with Bob
4.2.1. Example 1
Two image resolution alternatives are offered with 800x640 with
sar=1.1 having the highest preference
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It is also indicated that Alice wish to display video with a
resolution of 330x250 on her display
----
a=imageattr:97 send [x=800,y=640,sar=1.1,q=0.6] [x=480,y=320] \
recv [x=330,y=250]
----
In case Bob accepts the "recv [x=330,y=250]" the answer may look like
----
a=imageattr:97 recv [x=800,y=640,sar=1.1] \
send [x=330,y=250]
----
Indicating that the receiver (Bob) wish the encoder (on Alice's side)
to compensate for a sample aspect ratio of 1.1 (11:10) and desires an
image size on its screen of 800x640.
There is however a possibility that "recv [x=330,y=250]" is not
supported. If the case, Bob may completely remove this part or
replace it with a list of supported image sizes.
----
a=imageattr:97 recv [x=800,y=640,sar=1.1] \
send [x=[320:16:640],y=[240:16:480],par=[1.2-1.3]]
----
Alice can then select a valid image size which is closest to the one
that was originally desired (336x256) and performs a second offer/
answer
----
a=imageattr:97 send [x=800,y=640,sar=1.1] \
recv [x=336,y=256]
----
Bob replies with:
----
a=imageattr:97 recv [x=800,y=640,sar=1.1] \
send [x=336,y=256]
----
4.2.2. Example 2
Two image resolution sets are offered with the first having a higher
preference (q=0.6).
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----
a=imageattr:97 \
send [x=[480:16:800],y=[320:16:640],par=[1.2-1.3],q=0.6] \
[x=[176:8:208],y=[144:8:176],par=[1.2-1.3]] \
recv *
----
The x-axis resolution can take the values 480 to 800 in 16 pixels
steps and 176 to 208 in 8 pixels steps. The par parameter limits the
set of possible x and y screen resolution combinations such that
800x640 (ratio=1.25) is a valid combination while 720x608
(ratio=1.18) or 800x608 (ratio=1.31) are invalid combinations.
For the recv direction (Bob->Alice) Bob is requested to provide with
a list of supported image sizes
4.2.3. Example 3
In this example more of the SDP offer is shown. A complicating
factor is that the answerer changes the media payload type number in
the offer/answer exchange.
----
m=video 49154 RTP/AVP 99
a=rtpmap:99 H264/90000
a=fmtp:99 packetization-mode=0;profile-level-id=42e011; \
sprop-parameter-sets=Z0LgC5ZUCg/I,aM4BrFSAa
a=imageattr:99 \
send [x=176,y=144] [x=224,y=176] [x=272,y=224] [x=320,y=240] \
recv [x=176,y=144] [x=224,y=176] [x=272,y=224,q=0.6] [x=320,y=240]
----
In the send direction, sprop-parameter-sets is defined for a
resolution of 320x240 which is the largest image size offered in the
send direction. This means that if 320x240 is selected, no
additional offer/answer is necessary. In the receive direction four
alternative image sizes are offered with 272x224 being the preferred
choice.
The answer may look like:
----
m=video 49154 RTP/AVP 100
a=rtpmap:100 H264/90000
a=fmtp:100 packetization-mode=0;profile-level-id=42e011; \
sprop-parameter-sets=Z0LgC5ZUCg/I,aM4BrFSAa
a=imageattr:99 send [x=320,y=240]
a=imageattr:100 recv [x=320,y=240]
----
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Indicating (in this example) that the image size is 320x240 in both
directions. Although the offerer preferred 272x224 for the receive
direction, the answerer might not be able to offer 272x224 or not
allow encoding and decoding of video of different image sizes
simultaneously. The answerer sets new sprop-parameter-sets,
constructed for both send and receive directions at the restricted
conditions and image size of 320x240.
Note also that, because the payload type number is changed by the
answerer, the image attribute is also split in two parts according to
the recommendation in Section 3.2.2.
4.2.4. Example 4
This example illustrates in more detail how compensation for
different sample aspect ratios can be negotiated with the image
attribute.
We set up a session between Alice and Bob, Alice is the offerer of
the session. The offer (from Alice) contains the image attribute
below:
----
a=imageattr:97 \
send [x=400:16:800],y=[320:16:640],sar=[1.0-1.3],par=[1.2-1.3]] \
recv [x=800,y=600,sar=1.1]
----
First we consider the recv direction: The offerer (Alice) explicitly
states that she wish to receive the screen resolution 800x600,
however she also indicates that the screen on her display does not
use square pixels, the sar value=1.1 means that Bob must (preferably)
compensate for this.
So.. If Bob's video camera produces square pixels, and wish to
satisfy Alice's sar requirement, the image processing algorithm must
rescale a 880x600 pixel image (880=800*1.1) to 800x600 pixels (could
be done other ways).
... and now the send direction: Alice indicates that she can (in the
image processing algorithms) rescale the image for sample aspect
ratios in the range 1.0 to 1.3. She can also provide with a number
of different image sizes (in pixels) ranging from 400x320 to 800x640.
Bob inspects the offered sar and image sizes and responds with the
modified image attribute
----
a=imageattr:97 \
recv [x=464,y=384,sar=1.15] \
send [x=800,y=600,sar=1.1]
----
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Alice will, in order to satisfy Bob's request, need to rescale the
image from her video camera from 534x384 (534=464*1.15) to 464x384.
Neither part is required to rescale like this (sar may be ignored),
the consequence will of course be a distorted image.
5. IANA Considerations
Following the guidelines in [RFC4566], the IANA is requested to
register one new SDP attribute:
Attribute name: imageattr
Long form name: Image attribute
Type of attribute: Media-level
Subject to charset: No
Purpose: This attribute defines the ability to negotiate
various image attributes such as image sizes.
The attribute contains a number of parameters
which can be modified in and offer/answer
exchange.
Appropriate values: See Section 3.1.1 of RFCXXXX
Contact name: Authors of RFCXXXX
Note to RFC Editor: please replace "RFCXXXX" above with the RFC
number of this document, and remove this note.
6. Security Considerations
This draft does not add any additional security issues other than
those already existing with currently specified offer/answer
procedures.
7. Acknowledgements
The authors would like to thank the people who has contributed with
objections and suggestions to this draft and provided with valuable
guidance in the amazing video-coding world. Special thanks go to
Clinton Priddle, Roni Even, Randell Jesup, and Dan Wing. Thanks also
to Robert Sparks and Paul Kyzivat for the help with the last fixes to
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get the attribute work well with the offer/answer model.
8. Changes
Note to RFC Editor: please replace remove this section in its
entirety before publication.
The main changes are:
From WG -09 to WG -10
* Further clarified the issue that offer and answer may use
different PT numbers, additional section added.
* Additional typos fixed.
From WG -08 to WG -09
* Clarified the issue that offer and answer may use different PT
numbers
* Clarified that wild cards in send and recv direction should be
used with care and with total message size in mind
* Typos and unclear language fixe
From WG -07 to WG -08
* SHOULD changed to MUST in section "Offer/answer rules"
From WG -05 to WG -06 & -07
* Update based on AD review comments, no changes to fix issue
related to RFC2119 keywords
* Minor editorial changes
* Added extra example to use of attribute with SDPCapNeg
From WG -04 to WG -05
* Review based on WGLC comments
* ABNF improved
* Change use of RFC2119 keywords (MUST, SHOULD, MAY) to lowercase
in some sections
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